Oh, my bad, the plugin description is “This is a simple plugin that adds a recaptcha validation to the bbPress 2.0 topic creation and topic reply forum.” so it is not actually a recaptcha for the login page 😉

bbpress.org is hooked into wordpress.org so spam bots are handled by the WordPress team 😉

Have another look through the WordPress plugins for one that does what you require, there are a quite a few of them.